Integrating mud gas and cuttings analyses to understand local CGR variation in the Montney tight gas reservoir

Abstract

Distinctive areal change in the composition of produced hydrocarbon gas, in discordance with thermal maturity, has been observed in the Montney tight gas reservoir in the Western Canada Sedimentary Basin (Wood and Sanei, 2016). Predicting the distribution of hydrocarbon compositional variation, or condensate-gas ratio (CGR), has become key for exploration and development activities that aim to produce liquids-rich gas. The compilation of regional well production data allows for mapping of CGR variation, but not for direct comparison to rock properties. This study sought to define the controls on gas composition variation by examining the relationships between the reservoir rock properties of cuttings and the liquids content, or wetness, of mud gases. The study was conducted on two horizontal wells penetrating across a local produced-gas CGR anomaly in the Montney siltstone tight gas reservoir. The analysis of mud gas and drill cuttings from horizontal wells provides a high-resolution record of lateral changes in rock properties and in-situ gas composition. Before comparison to cuttings properties, mud gas data was first quality-controlled and normalized to remove the influence of changes in drilling conditions, such as rate of penetration (ROP). Mud gas data from fifteen local wells was then correlated to produced-gas composition to confirm the applicability of mud gas wetness as a proxy for CGR. Permeability indices of drill cuttings were analyzed by laboratory NMR and MICP, while mineralogy was determined by QEMSCAN. Concentration of Rock-Eval total organic carbon (TOC) was used to calculate solid bitumen saturation, which is said to influence the permeability of Montney type reservoirs (Wood et al., 2015). A positive correlation was found between production-tested gas CGR and mud gas wetness ratio. Mud gas wetness was negatively correlated to cuttings permeability, which was negatively correlated to TOC-derived solid bitumen saturation. These results support the suggestion that up-dip methane migration occurred along pathways of high permeability and low bitumen saturation. Mud gas wetness and cuttings permeability indices were confirmed to be effective at determining and predicting trends in the liquids content of hydrocarbon gas.